Should I keep a used Omamori?

I have recently received from a friend old and used omamori after she had learnt that I love keep omamoris.

So I was just wondering, am I allowed to keep them?
or should I return them back to the Temples in Japan?

You can keep them and look after well.
Omamori works in a mysterious way.

After a year of purchasing your omamori, it is supposed to be returned to the shrine/temple you bought them from. But a shrine tells me that you can also return them to any shrine/temple.

At temples and shrines, there is a certain area to place your used omamori. They will burn them for you while giving prayers.

It is said that omamori basically lose power after a year, and I also recall hearing it's not proper to use used omamori you got from someone else. But I suppose it all depends on your faith.